Multiple use of single qpid::management::ManagementAgent::Singleton confusing
and error-prone
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Key: QPID-1843
URL: https://issues.apache.org/jira/browse/QPID-1843
Project: Qpid
Issue Type: Bug
Components: C++ Broker, Qpid Managment Framework
Affects Versions: 0.5
Reporter: Steve Huston
Currently the qpid/agent/ManagementAgent.h file declares the
qpid::management::ManagementAgent pure virtual class. Places where management
agent functionality is desired must derive a class from ManagementAgent at a
minimum. This is fine. However, there's also a non-virtual
ManagementAgent::Singleton class defined in ManagementAgent.h. Current practice
for this arrangement requires each place where a ManagementAgent-derived class
is defined to also reimplement (including static class members)
qpid::management::ManagementAgent::Singleton, the implementation of which
manages the new class derived from ManagementAgent.
This causes a problem because the ManagementAgent::Singleton class's member
functions must be exported from the implementing DLL. However, the implementing
DLL is not known when the class is declared and, in fact, there may be multiple
implementations. For example, the qmf-agent has an implementation, and so does
the broker. Currently, Singleton is marked as exported from the QMF-agent
library. Thus, when broker is built, it sees both locally-defined and imported
Singleton symbols. Apparantly, g++ is ok with this and just takes the local one
(even if that's not what you want, but I digress...).
It wouldn't work to just avoid exporting the symbols, because they are needed
externally. For example, the acl plugin needs to get the broker's singleton
pointer.
One idea is to simply avoid the use of Singleton. If a instance of a
ManagementAgent-derived class is desired, just instantiate one. Or add a method
to the subclass to manage a singleton. Or whatever works for the particular
class's use cases. The point is to push the lifetime management to the
agent-derived class.
In the case of the broker, the ManagementBroker (derived from ManagementAgent)
could simply be used (or not, depending on the disable-management setting), and
a method be added to Broker to obtain the ManagementAgent pointer, which could
be a refcounted pointer since it's often saved. Then the, for example,
ManagementBroker could export the symbols it needs, if any, in addition to the
getMgmtAgent (or whatever) method.
Any other ideas on this? I can start working on the changes when we decide on a
way to go.
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